Epidemiological studies suggest that repetitive loading is of importance in the development of back injuries[unreadable] leading to early disc degeneration. In vitro studies have confirmed the biological plausibility of injuries[unreadable] occurring from excess loading. However, in vitro studies are limited in their ability to improve our[unreadable] understanding of the complex relationships that exist between the large numbers of parameters needed to[unreadable] describe the effects of single and repetitive load applications and how disc tissues break down over the[unreadable] course of many years of exposure. We believe that an improved understanding of these complex[unreadable] relationships, namely how failure progresses under cyclic loading in discs with different grades of[unreadable] degeneration and how facet degeneration changes the load path within the disc, can best be achieved by[unreadable] numerical techniques such as the Finite-Element Model (FEM). Our studies to date have shown the[unreadable] potential of FE models to study disc responses to a variety of loading conditions. At this time the FEM is[unreadable] one of a healthy disc, while in actual life disc degeneration occurs early and may alter the response of the[unreadable] disc to loading. The already developed and validated poro-elastic FEM of a healthy disc will be modified to[unreadable] reflect various stages of disc degeneration and will be used to test the following hypotheses: (1) Disc[unreadable] failure will occur after fewer repetitions and at lower load levels and will propagate more rapidly in[unreadable] degenerative discs compared to healthy discs. (2) The existence of fissures, clefts and cracks in the disc[unreadable] will cause redistribution of the stresses and cause abnormal motions resulting in further enhancing the[unreadable] failure of disc material. (3) As the loading pattern on the discs and the disc structure changes, areas of high[unreadable] stress concentrations will occur in the facet joints leading to failure of the facet joint cartilage.[unreadable] These hypotheses will be addressed with the help of the following specific aims: (1) Determine the[unreadable] failure progression due to cyclic loads in a healthy disc and in discs of different grades of degeneration[unreadable] using a more refined poro-elastic FEM which includes re-bar elements to represent annular fibers and uses[unreadable] "user-defined" material models to determine failure progression continuously as the cyclic loading[unreadable] progresses. (2) Determine the effect of including fissures in the outer and inner annulus and cracks in the[unreadable] endplates on the progression of failure in degenerated discs. Poro-elastic parameters such as porosity and[unreadable] permeability of the disc components will also be modified to reflect these failures. (3) Determine the loads[unreadable] on facet cartilage in discs of different grades of degeneration.